Leaf spring devices made from leaves which are each a plate-shaped steel (steel plate) are often used in suspension apparatuses of vehicles (particularly, heavy duty vehicles).
In recent years, there has been a demand for a reduction in the weight of vehicles. In response to this, there is a demand for a reduction in the weight of the leaf spring devices. Therefore, a high stress design has drawn attention in which: a material is heat-treated to have a higher hardness and thus have a higher fatigue strength (see Patent Literature 1).